A film made of a fluororesin, a fluororesin composite or the like is light in weight and excellent in weather resistance, mechanical strength, etc. and therefore, used as the roof, the outer wall etc. in facilities such as sports facilities (such as a swimming pool, a gymnasium, a tennis court and a football ground), warehouses, halls, exhibition halls and horticultural facilities (such as a horticultural greenhouse and an agricultural greenhouse) (for example, JP-A-3-188132, JP-A-63-222852, JP-B-8-5976). Hereinafter, a roof, an outer wall, facilities or the like using a film will be referred to as a membrane structure. Further, a film to be used for a membrane structure will be referred to as a film for a membrane structure.
However, the film made of a composite of glass fibers and a fluororesin such as a polytetrafluoroethylene has the following problems.
The film has insufficient transparency and has a low sunlight transmittance, and is thereby unsuitable as a film for a membrane structure for horticultural facilities or for sports facilities in which glass grows.
The film has insufficient transparency, through which the surrounding landscape cannot be viewed, and thus when it is used for a hall, an exhibition hall or the like, the interior of such a structure will be insufficient in wide open feeling.
On the other hand, a film made of a fluororesin having high transparency such as an ethylene/tetrafluoroethylene copolymer has a high sunlight transmittance and is thereby suitable for a film for a membrane structure to be used for horticultural facilities (JP-B-08-5976), sports facilities in which grass grows, exhibition halls, etc.
However, even when a film made of a fluororesin having high transparency is used, its transparency decreases if the surface of the film is fouled. Accordingly, such a film is desired that dirt attached to the film spontaneously decomposes to prevent a decrease in transparency. As such a film, a film comprising a photocatalyst layer on a film substrate has been known. However, this film has such a problem that when it is left to stand outside for a long time, the film substrate itself undergoes oxidative decomposition by the photocatalyst, i.e. the film has insufficient weather resistance. Further, the film has such a problem that the adhesion of the photocatalyst layer to the film substrate is insufficient.
As a technique to suppress oxidative decomposition of the substrate by the photocatalyst and to improve adhesion of the photocatalyst layer, a technique of providing an interlayer between a photocatalyst layer and a substrate layer has been known.
(1) A building material which comprises an interlayer made of a silicone resin interposed between a coating film of e.g. a fluororesin formed on a substrate of e.g. aluminum and a photocatalyst layer (Japanese Patent No. 3523787).
(2) A moisture condensation-preventing film which comprises an interlayer made of a silicone resin interposed between a film substrate of e.g. a fluororesin and a photocatalyst layer (JP-A-2005-271340).
However, when the building material (1) is left to stand outside for a long time, the coating film of e.g. a fluororesin will undergo oxidative decomposition, since the interlayer itself is likely to undergo oxidative decomposition by the photocatalyst. Further, once the interlayer undergoes oxidative decomposition, cracking or whitening will occur in the photocatalyst layer.
The moisture condensation-preventing film (2) is assumed to be bonded to another substrate by means of an adhesive layer, and accordingly, the film substrate is thin and the film has insufficient mechanical strength. Further, as the silicone resin for the interlayer is hard, if the film is left to stand outside for a long time without being bonded to another substrate, the interlayer cannot follow the elongation of the film substrate, thus resulting in cracking in the interlayer and the photocatalyst layer.
As mentioned above, the building material (1) and the moisture condensation-preventing film (2) are insufficient in weather resistance and durability. Thus, a film for a membrane structure excellent in weather resistance will not be obtained even when the interlayer in (1) or (2) is applied as it is to the film for a membrane structure comprising a photocatalyst layer on a film substrate.